The present invention relates to a friction vacuum pump containing at least one turbomolecular pump stage, a screw pump stage connected on the delivery side of the pump, and a filling stage arranged between the turbomolecular pump stage and the screw pump stage.
In turbomolecular pumps with downstream screw pump stages, which are also called compound pumps, the pumped gas needs to be transferred from a pump chamber having a relatively large volume in which the axial compressor stages are located, into a pump chamber (pumping slot) having a relatively small volume in which the screw is located. Known designs for this transitional area have the disadvantage that the flow will breakdown. This will significantly impair the pumping capacity of the pump.
From DE-A-196 32 874 it is known to provide between the turbomolecular pump stage and the downstream screw pump stage, a filling stage which is equipped with blades. The production of such an intermediate stage is involved. Moreover, the presence of the blades will hamper assembly.
It is the task of the present invention to considerably simplify the design of the filling stage without impairing the effective supply of gas into the screw pump stage.
This task is solved through the present invention by designing the filling stage as a centrifugal stage. Components of the centrifugal stage are rotating ridges which are located at the level of the intake area of the screw pump stage. The centrifugal pump has the effect of deflecting and compressing the gas ejected by the turbomolecular pump stage and supplying the gas into the pump chamber of the screw pump. The gas flow is substantially continuous so that the flow will no longer breakdown thereby impairing operation.
Owing to the fact that the centrifugal stage deflects the gas flow outwards, there exists the possibility of selecting a relatively large diameter for the pumping slot of the screw pump stage, so that the rotating pumping surface of the screw pump stage has a high circumferential speed.
If the inside diameter of the stator at the outside of the screw pump stage is greater than the outside diameter of the rotor of the turbomolecular pump stage, then there exists, provided that the stator of the turbomolecular pump consists of spacing rings and blade half-rings in a basically known manner, the possibility of being able to assemble or disassemble the stator of both pump stages without the need to assemble resp. disassemble the rotor of the turbomolecular pump stage. These dimensions will allow the rotary system of the compound pump to be balanced first, followed by subsequent assembly of the stator components.